Jack Lynch Tunnel
| Jack Lynch Tunnel Irish: Tollán Sheáin Uí Loingsigh | |
| County Cork | |
|---|---|
 The interior of the southbound tube | |
| Location | |
| Carrying: | N40 road |
| Beneath: | River Lee |
| Location | |
| Grid reference: | W73077200 |
| Location: | 51°53’60"N, 8°23’31"W |
| Structure | |
| Length: | 2,000 feet |
| History | |
| Built 1995 – 1999 | |
| Information | |
| Operated by: | Egis Road and Tunnel Operation Ireland |
The Jack Lynch Tunnel is an immersed tube tunnel and an integral part of the N40 road network in the City of Cork, County Cork. It is named after a former Taoiseach, Jack Lynch, a native of Cork.
The tunnel takes the road under the River Lee. North of the tunnel, the ring-road joins the M8 motorway towards Dublin (north) and N8 road to the city centre (west), with the N25 commencing east to Waterford.
The tunnel was completed in May 1999, and carried nearly 40,000 vehicles a day as of 2005. This number rose further as the N40 ring-road's upgrades progressed, with the opening of the Kinsale road roundabout flyover in 2006 and subsequent upgrades to the Sarsfield Road and Bandon Road Roundabouts. Traffic in 2016 was approximately 63,000 vehicles a day up from 59,000 in 2013.
The tunnel has two cells, each with two traffic lanes and two footpaths, and a central bore for use in an emergency only. Pedestrians and cyclists are expressly forbidden from using the tunnel.[1] The exclusion of cyclists has been somewhat controversial as the feeder road is a dual-carriageway and so is open to cyclists, but the by-law is applied because of space limitations and the obvious danger of cyclists in an enclosed tunnel.
History
The idea of a crossing of the River Lee downstream of the city centre came from civil engineers employed by Cork Local Authorities and the central government's Department of the Environment in the late 1970s. Cork's suburbs were expanding and traffic was rising as car ownership increased, but the city centre's street plan, laid out in the late Middle Ages, was ill-equipped to cope. The engineers reasoned that the congestion in the city centre and its radial routes was quickly reaching intolerable levels.
No road development in Ireland prior to that date had required such a large investment, and therefore the plan met with some opposition on the grounds of cost. In 1980, Cork Corporation commissioned DeLeuw Chadwick O’hEocha, engineering consultants, to undertake a feasibility study of options for a major highway crossing of the River Lee downstream from the city centre. A team led by J.D. Shinkwin, Director of DeLeuw Chadwick O’hEocha, performed the study. The first stage of the report established that the crossing should be located in Dunkettle, rather than at Tivoli, closer to the city centre. The second stage endorsed an immersed tube tunnel as the preferred scheme versus a bridge (either fixed or opening span). The primary reasons were that a tunnel would have a lower construction cost relative to its utility, shorter approach gradients, lower environmental impact and no effect on shipping once built. While the construction costs for a two lane tunnel were marginally higher than for a two lane high-level bridge, the steep upward ramp for a high-level bridge would slow down cars and trucks as they climbed the bridge, thus reducing its peak capacity dramatically versus a tunnel or, alternatively, necessitating the addition of a climbing lane with significant additional costs. The other alternative considered by the Steering Committee was an opening span bridge. While this solution would avoid the ramp problem of a high-level bridge, traffic would halt whenever the bridge had to open. Shipping volumes into Cork's port area was rising steadily in the 1980s, which meant that an opening span bridge was becoming less attractive every year.
In 1989, the government recommended a further feasibility study to consider the need for a crossing and the technical, economic and operational aspects of the various crossing options. The study confirmed the findings of the original Feasibility Study. A further Public Inquiry was held, with hearings in July and December 1990, which considered further the alternative of a four-lane crossing rather than the planned two lane crossing.
In 1992, the Minister approved a Bridge Order for a four-lane immersed tube tunnel. In the intervening time, the Irish government had also established the National Roads Authority to advance the long-term development of the national roads network. Declan O'Driscoll was appointed the Chief Engineer of that authority and together with J.D. Shinkwin, Director of Ewbank Preece OhEocha, and W.A. Fitzgerald, Cork City Engineer, formed an informal Technical Steering Committee for the Tunnel Project.
Financing and contracting
The IR£70 million design and construct contract was awarded by the Cork Corporation on behalf of the National Roads Authority.
In the early 90s, Ewbank Preece OhEocha (formerly DeLeuw Chadwick OhEocha, and later to become part of Mott MacDonald) supervised a major geotechnical investigation, conducted a hydraulic study of the river, and carried out an environmental impact study. Ewbank Preece OhEocha, in association with Symonds Travers Morgan, produced a conceptual design and tender documents for Cork Corporation on a design and construct format based on geometric and performance specification.
Construction
Construction involved the excavation of a large casting basin where the tunnel elements or pieces were constructed. After construction of elements was complete, the casting basin was filled with water and joined to the adjacent River Lee, each element was floated out and sunk into position into a carefully dredged river bed. The road surface was laid and the tunnel opened for traffic in 1999.
The chosen method of construction was the immersed tube technique. In this method, a trench is dredged in the bed of the water channel. Tunnel sections are constructed in the dry, for example in a casting basin, a fabrication yard, on a ship-lift platform or in a factory unit. The ends of the section are then temporarily sealed with bulkheads. Each tunnel section is transported to the tunnel site – usually floating, occasionally on a barge, or assisted by cranes. In the Jack Lynch Tunnel, the 610-metre-long reinforced concrete immersed tube tunnel is made up of five elements, each around 122 metres long, 24.5 metres wide and 8.5 metres high. The northern approach was formed by a 120-metre-long floated open 'boat' section – the first of its kind.
Excavation and dredging
Dredging International was the chosen dredging contractor, and the value of the dredging contract is stated by them at €12.4 million. The scope of works consisted of dredging the main tunnel trench and subsequently backfilling the completed construction with sand and gravel. The contract provided also for restoration of the riverbed profile and the addition of a rock protection layer. Between May 1996 and March 1999, the company excavated 785,000 tonnes of silt and alluvium and 300,000 tons of sand and gravel.
The trench was dredged primarily by a series of specialised vessels, in several stages and with different equipment. Much of the main trench for the Lee Tunnel was excavated by the backhoe dredger "Zenne". Two barges were used to transport the dredged material 12 miles downriver and from there to a disposal site four miles offshore.
Constructing the immersed tube
Building the tunnel structure required the pre-casting of 5 tunnel segments and an open boat unit. These were constructed in a casting basin located partially on the line of the tunnel south of the river at Mahon. Each segment weighed about 27,000 tons.[2] Contractors Tarmac Walls JV engaged RMD Kwikform, a global construction formwork and shoring solutions company, for the formwork on the tunnel and open section elements of the river crossing.[3]
Each box section comprised two dual lane traffic tubes 32 feet wide and a twin walled central tube 4 feet wide for services and emergency access. The base slab 4 feet thick was cast first, followed by the 50-centimetre-thick central walls. The outer walls and roof were cast together in a single operation, in six nominal 66-foot lengths, using special travelling formwork. This was struck inside the formed section, moved along to the next length and then jacked up into position, each within a 72-hour cycle. The inside shutters for the outer walls were vertical steel section panels and Alform Beam walers, which were attached to the top slab of the special steel traveller, while the outer shutters were crane handled vertical steel section panels with Alform Beam or steel channel walers.
The open top boat unit, measuring about 130 feet wide (including two wings 24 feet at its widest point), 394 feet long, and up to 33 feet high, was cast in two operations using standard formwork.
An immersed tube tunnel of similar length, the Limerick Tunnel, has been constructed in Limerick,[4] as part of Limerick's south ring road.
Outside links
References
- ↑ "Home > Jack Lynch Tunnel". Jacklynchtunnel.ie. http://www.jacklynchtunnel.ie.
- ↑ "Traffic Management in Cork". CGTA. http://www.iol.ie/~cgta/cork.htm.
- ↑ "River Lee Immersed Tunnel". rmdkwikform.net. http://www.rmdkwikform.net/rmd_projects/river_lee_tunnel.html.
- ↑ Limerick Tunnel : What is the Limerick Tunnel?